Poly ether ketones have a variety of useful properties, such as excellent electrical insulating and mechanical properties at high temperature, high strength, toughness and resistance to heat and chemicals. Such polymers may be amorphous or semi-crystalline. Both types usually exhibit high glass transition temperatures (Tg), while the semi-crystalline forms also exhibit high melting temperatures (Tm). Amongst these polymers, the poly (ether ketone ketone) family is of particular interest for use in preparing biomedical implants and implant materials due to their excellent mechanical properties, chemical inertness and resistance to stress cracking. The same materials are also useful in aerospace and many other wide-ranging industrial applications including the preparation of thermoplastic composites.
Common terminology involves naming such polymers by reference to the structure of the repeating unit (as is standard in polymer chemistry) with families being named according to the sequence of ether (symbolised by “E”) and ketone (symbolised by “K”) linkages in the repeat units. For example, polymers consisting essentially of the repeating unit: —R—O—R—C(═O)—R—C(═O)— would be referred to as “PEKK”.
For in vivo use, PEKK materials must further meet the requirement of biocompatibility which, in turn, demands a high level of purity of the basic polymer. Ideally, it is required that these can be produced in essentially pure form on a large scale in the absence of substantial amounts of unreacted monomers, catalyst residues or other reaction components or contaminants. These impurities can also result in melt instability of the polymers during processing. This can be a serious problem during the preparation of composite materials as the instability can lead to property changes during manufacture that may impact performance in-use. For structurally critical applications, such as in aerospace, this is highly undesirable.
Aromatic polyetherketones are commonly prepared by either a nucleophilic or electrophilic polymerisation. For example, the polymer PEEK as supplied by Victrex Plc is understood to be synthesised by a high temperature nucleophilic process as depicted:

This type of reaction can be referred to as an ether-forming reaction as the result of the reaction is the formation of an ether linkage.
Alternatively, poly ether ketones such as PEKK may be formed using an electrophilic polymerisation as depicted. This is also commonly referred to as the Friedel-Crafts method.

This type of reaction can be referred to as a ketone-forming reaction as the result of the reaction is the formation of a ketone linkage.
Unlike the nucleophilic reaction, the electrophilic or Friedel-Crafts reaction may be conducted at elevated temperature as disclosed in U.S. Pat. No. 4,816,556 or at ambient or sub-ambient temperature as taught in U.S. Pat. No. 4,841,013. Typical reaction media for these reactions include the reactants (i.e. the monomers), a catalyst (or promoter, e.g. K2CO3 or AlCl3) and a suitable solvent.
The nucleophilic reaction is a solution reaction in that the growing polymer chain is maintained in a reactive state by the polymer remaining in solution (e.g. in diphenyl sulphone at elevated temperature). In contrast, the electrophilic reactions are not true solution reactions as the polymer is inclined to precipitate out as the chain length grows. Unlike a normal precipitation, the particles aggregate forming an intractable mass. The mobility of this mass is maintained in the process of U.S. Pat. No. 4,816,556 by the use of elevated temperatures and by the incorporation of a Lewis base in the process of U.S. Pat. No. 4,841,013 to form a deformable complexed gel structure in which there remains sufficient end-group mobility to enable polymerisation to continue.
Both the high temperature electrophilic and nucleophilic processes can produce products that exhibit poor melt stability resulting from side reactions. The very high temperatures used in the nucleophilic process can result in the scrambling of the ether and ketone linkages resulting in products in which the linkages are not as ordered as theory would predict or as desired. The high temperatures associated with these two processes can also promote side reactions and the formation of gels (probably cross-linked particles) that lead to melt instability in the final product. These gels may appear as discoloured inclusions. This makes the production of materials having a fine structure (e.g. fibres and thin films) difficult or, in some cases, impossible.
U.S. Pat. No. 4,912,181 discloses a low temperature electrophilic process which results in the formation of a complexed gel (with AlCl3) and which requires the use of specialised equipment to facilitate the extraction of the final product from the reaction equipment. When practised correctly, this low temperature Friedel-Crafts process can produce a final product that is neither scrambled nor contaminated with side reaction products or imperfections (e.g. cross-linked gels) thus enabling the production of fine products such as fibres without the need for melt filtrations, or other gel removing protocols, prior to processing.
Typically, the organic moieties connecting the ether and ketone linkages in polyetherketones are aromatic units which are in themselves 1,4 or 1,3-disubstituted (1,2-disubstitution is known but is unusual). All 1,4 substituted polyetherketones exhibit high levels of crystallinity. In addition, the Tm of the polymers increase as the ratio of ketones to ethers is increased. Thus the Tm of 1,4-PEEK is around 345° C. and that of 1,4-PEKK is around 395° C. whilst both exhibit ultimate levels of crystallinity of between 35% and 40%. Although 1,4-PEKK can be readily synthesised by either of the aforementioned electrophilic processes, the high Tm, makes synthesis by the nucleophilic process difficult as the polymer is reluctant to remain in solution long enough for high molecular weight product to be formed. It is well known that the Tm, of all poly ether ketones can be manipulated by the incorporation of 1,3 units (and 1,2) in the structure such that suitable mixtures of 1,4 and 1,3 units can lead to the production of amorphous products. Whilst all 1,4-PEKK is difficult to melt process it can be solution processed from solvents such as concentrated sulphuric acid.
In order to facilitate the electrophilic synthesis of PEKK the reaction may be undertaken in strongly acidic solvent systems such as HF/BF3 (see e.g. U.S. Pat. No. 3,956,240) or perfluoroalkyl-sulphonic acids (see e.g. U.S. Pat. No. 4,396,755). However, these solvent systems are highly corrosive and thus present handling problems. Alternative commercial methods for preparing 80:20 PEKK (80% 1,4: 20% 1,3) include the high temperature electrophilic process disclosed in U.S. Pat. No. 4,816,556. However, as mentioned above, this high temperature process can result in the production of an unstable product (high temperatures increase the likelihood of side reactions) containing imperfections (e.g. cross-linked gels), making it quite unsuitable for certain end uses, e.g. in biomedical implants, in producing articles having a fine structure and in critical aerospace composites.
U.S. Pat. No. 5,734,005 describes a modified Friedel-Crafts synthesis of polyetherketones using polymeric Lewis bases leading to the formation of high molecular weight products. It is believed that the use of these Lewis acid/Lewis base controlling agents alters the solubility parameter of the solvent such that the polymer complex remains in solution longer and yet still permits polymer chain mobility required for the production of high molecular weight products. The disadvantage of this process and that of the complexed gel products disclosed in U.S. Pat. No. 4,912,181 is the high cost of the specialised equipment required to handle the polymer gel complex. Additional disadvantages include the large volumes of water required to decomplex and work up the polymer, owing to its very low bulk density, and the difficulty in recovering most of the solvent or Lewis base.
The process of U.S. Pat. No. 4,841,013 employs protic controlling agents for the preparation of poly ether ketones, in particular PEKEKK. The authors note that it cannot be reliably predicted whether a particular controlling agent will act as a dispersant, this being dependent on other reactants and conditions. This is confirmed by the present inventor's own findings. In fact, the present inventor has found that for every 10 polymerisations carried out as described in U.S. Pat. No. 4,841,013 for PEKEKK production, on average only 1 results in the formation of a finely dispersed polymer product. The remaining polymerisations fail to provide the desired end product due to gel, or partial gel, formation. Efforts to address this in U.S. Pat. No. 4,841,013 include the incorporation of a second non-polar solvent such as cyclohexane. However, even this is found to be unreliable. Such a level of unpredictability is unacceptable, especially in a commercial process.
Whilst U.S. Pat. No. 4,841,013 discloses the production of PEKK using butanol as controlling agent, this may lead to alkylation of the polymer chain resulting in the formation of an unstable product. Moreover, the butanol is not recoverable.
U.S. Pat. No. 4,912,181 discloses a process by which the polymer complexed gels may be handled. Initial mixing and reaction is carried out in one reactor and prior to gel formation the reaction mixture is transferred to a tubular reactor where gelling occurs. On completion of polymerisation the gel is extruded from the tube into a hammer mill where decomplexation takes place in the presence of water leading to isolation of the polymer. Although high quality products can be manufactured in this way, the process requires special equipment and thus suitably adapted large scale plants for commercial production, making this method costly.
To date, despite its advantageous properties, the issues outlined above have made the large scale production of highly pure and melt-stable PEKKs problematic.